The present invention relates to a split engine.
In an internal-combustion engine in which the load of the engine is controlled by a throttle valve, a specific fuel consumption deteriorates as the degree of opening of the throttle valve is reduced. Consequently, in order to improve the specific fuel consumption, a split engine as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-69736 has been devised in which, when the engine is operating under a light load, part of the cylinders of the engine are not fired while the remaining cylinders operate under a heavy load. As is illustrated in FIG. 1, in this type of engine, the cylinders are divided into a first cylinder group A and a second cylinder group B, and a first intake manifold 1 and a second intake manifold 2 are connected to the first cylinder group A and the second cylinder group B, respectively. The first intake manifold 1 and the second intake manifold 2 are connected to the outside air via a common throttle valve 3, and a shut-off valve 4 is arranged in the air inlet of the first intake manifold 1. The first intake manifold 1 and an exhaust manifold 5 are interconnected to each via an exhaust-gas recirculation passage 6, and a recirculation control valve 7 is arranged in the exhaust-gas recirculation passage 6. In the split engine, when the engine is operating under a light load, the injection of fuel by fuel injectors 8 is stopped, and the shut-off valve 4 is closed. In addition, the recirculation control valve 7 is open, and the cylinders of the second cylinder group B operate under a heavy load. Contrary to this, when the engine is operating under a heavy load, the shut-off valve 4 is open, and the recirculation control valve 7 is closed. In addition, fuel is injected by all of the fuel injectors 8 and 9, and, thus, all of the cylinders are fired.
As mentioned above, when the engine is operating under a light load, the shut-off valve 4 is closed, and the recirculation control valve 7 is open. As a result, since the exhaust gas is recirculated into the first cylinder group A via the exhaust-gas recirculation passage 6, it is possible to prevent pumping loss in the first cylinder group A from occurring. In addition, at this time, since the cylinders of the second cylinder group B operate under a heavy load, it is possible to improve the specific fuel consumption.
In such a split engine, however, at the time of changing the number of cylinders to be fired, control of the engine is the most difficult and various problems occur. For example, in the split engine illustrated in FIG. 1, when the operating state of the engine is changed from a light load to a heavy load, first, the recirculation control valve 7 closes. Then, the shut-off valve 4 abruptly opens, and the injection of fuel into the first cylinder group A is started. However, when the opening speed of the throttle valve 3 is low, that is, when the engine is gently accelerated, if the shut-off valve 4 abruptly opens, the abrupt increase in the output torque of the engine provides a chock for a driver. This results in a problem in that the drivability of a vehicle deteriorates. Contrary to this, when the opening speed of the throttle valve 3 is high, that is, when the engine is abruptly accelerated, if the shut-off valve 4 opens after the recirculation control valve 7 closes, it takes a long time until the shut-off valve 4 opens after the accelerating operation of the engine is started. This results in a problem in that a good accelerating operation cannot be obtained.